They're mostly rock and ice, though Ganymede, the biggest, may have an iron core.

它们大多是岩石和冰构成，最大的木卫三可能有一个铁核。

Naturally occurring magnets aren't powerful enough to lift up huge slabs of steel, but wrap an iron core with wires and run a current through and voila, you can create a powerful electromagnet that can lift cars.

自然产生的磁铁不足以举起巨大的钢板，但用电线包裹铁芯并让电流通过， 瞧，你可以制造一个可以举起汽车的强大电磁铁。